RAM connection checks Panel web shear for Axial load

This document is somewhat old.  When it came out we asked AISC for clarification, and as long as the forces are in equilibrium we are satisfied to consider the beam axial forces. The following is the question and response with a few names redacted.  If you decide to neglect it, you can always override the beam axial forces to 0. 

Enter your question below:

Hello AISC I would really appreciate you can help me with a question regarding web panel zone shear strength calculation. Please refer to the steel quiz published in the AISC Modern Steel Construction Magazine on June 2014. In the first point of the quiz it ask about: (a) What is the story shear, Vc? (b) How does the axial load affect the web panel zone shear check? (c) What is the required web panel zone shear strength, Vp? According to the answer to Point 1(c) in this quiz, axial force in the beam should not affect the web panel zone shear (should be ignored). That generate some doubts about it because in the AISC manual we can find recommendations that the required shear, Ru or Ra, is primarily transferred through the beam-web connection and the moment can be resolved into an effective tension-compression couple where the required force at each flange, Puf or Paf. Axial forces, if present, are normally assumed to be distributed uniformly across the beam flange cross-sectional area. Should the beam axial load should be considered to web panel zone shear strength calculation? Thank you very much for your help in advance.

Answer:

There are inherent problems with trying to condense technical concepts into one page questions and answers. The quiz question you are addressing I think it is valuable in that it conveys a simplified rule that can be followed in design. The rule is more concisely stated below. It is not intended to be a thorough and rigorous treatment of the issue.
I notice you work for Bentley so I will state that this is a rule commonly applied by humans who apply engineering judgment and knowledge to less than ideal and complete design information that is often provided in contract documents. The loads provided are often not from consistent load combinations and may not be actual loads at all. If you have full knowledge of the structure and its loads, then there is no need to apply such rules as you can work out the actual demands without them.
Does the beam axial load should be considered to web panel zone shear strength calculation? The answer could be Yes or No. So I understand your confusion. The question is intended to demonstrate that a moment from the beam caused by lateral loads will cause panel zone shear. The necessary reaction to the moment at the column supports (the story shear) will act in the opposite direction as the story shear, thereby reducing the shear that must be considered at the panel zone.
This involves a bit of semantics. The Specification defines the panel zone as “Web area of beam-to-column connection delineated by the extension of beam and column flanges through the connection, transmitting moment through a shear panel”. Take the sketch from the quiz and ignore the moment and axial load on the column. Distribute the 20 kips axial load on the beam as 10 kips to each flange as you suggest. The column is not in equilibrium, so you must apply 20 kips in the other direction distributed in some manner. Let’s just assume all 20 kips is resisted at the bottom cut through the column.
Does this arrangement produce a shear on the column? Yes.
Why then is it not considered in the Steel Quiz example? The panel zone check is used to determine whether or not the column must be reinforced locally (at the panel zone) by a doubler to resist the shear from the moment. In the centerline (computer) models typically assumed in design the column will have already been designed for the shear caused by the beam axial loads and for what is referred to as the story shear in the quiz. What is called the panel zone shear will not have been considered in the centerline model. This is the difference.
Typically, it seems the axial shown would also be reflected in the story shear. In which case, it is accounted for as you assert it must be. However, it is conceivable that the axial (or some part of it) is not accounted for in the story shear that it serves some function other than resisting the moment from the beam. This is perhaps clearer if you take the sketch from the quiz and draw a beam opposite the one shown. Now apply a moment to the beam acting in the same direction as that on the first beam (as would result from lateral loads on the frame). Now draw a 20 kip axial load on the new beam acting in the opposite direction as the first. This axial load causes no shear on the column. This was really the point we were trying to make.
One cannot really state whether the quiz question and answer is correct or not, since the figure shown only shows the local condition and does not represent a complete and statically admissible free-body diagram. I find I can complete the diagram with forces that support the answer provided or with forces that do not support the answer provided.
From a practical standpoint, if I am designing a moment connection and I have been given a moment that causes shear in the panel zone then I know this effect will always be counter-acted by what we are calling the story shear. If I also have an axial load, this axial load (if it is from the same load case as the moment) must either be part of the story shear or must have no effect on the panel zone shear.
If you are uncertain as to how to proceed for any given situation in practice, you should draw a complete free-body diagram (or a series of them to cover all possibilities).

Seth san,

Thank you for your prompt reply and the elaboration.

Seth Guthrie said:

Let’s just assume all 20 kips is resisted at the bottom cut through the column.

The assumption mentioned in the AISC reply cannot be generalized. As explained above, it depends on structure configuration & lateral load pattern. But that doesn't mean RAM connection should consider Axial force in Panel Web Shear check by default. Instead, I expect Bentley to include it as an option in the next update of the software. So that it will be highlighted to user and choose whether to ignore Axial load.

Seth Guthrie said:

If you decide to neglect it, you can always override the beam axial forces to 0. 

If Bentley includes this option in RAM connection along with above explanation in the "Help" it would be useful for the users who do not have read above reply and they will either end up designing very conservative connection or doubting the RAM connection calculation itself.